1. Field of the Invention
The present invention relates to rock drill bits, and more particularly to a rock drill bit assembly for releasably mounting drill bit cutters.
2. Related Art
Polycrystalline Diamond Compact (PDC) drill bits used for rock drilling are currently manufactured using PDC cutters that are cast into, or brazed onto, a tungsten carbide bit body. Alternatively, the cutters are sometimes brazed onto a stud that is then inserted into a steel bit body. However, these bits are expensive to manufacture and for several reasons are limited in performance.
Currently, field adjustment and/or replacement of worn or damaged PDC cutters is extremely difficult and sometimes impossible. As PDC cutters wear, the efficiency of the bit is reduced. If the cutters are brazed into a bit body, these cutters cannot be subsequently rotated within their cutter seats-preventing utilization of the entire perimeter of the cutter as a leading edge. In addition, premature failure of any single PDC cutter, if not replaced, results in accelerated wear and/or failure of the entire bit.
Another problem with the brazing process conventionally used to affix cutters to the bit body is that it requires very high temperatures. The heat generated during this process results not only in weakening of the surrounding bit structure itselfxe2x80x94causing greater likelihood of cutter attachment failure during usexe2x80x94but also results in the likelihood of heat damage to the cutter""s synthetic diamond structurexe2x80x94causing greater wear and premature need for replacement.
U.S. Pat. No. 3,749,190 teaches a rock drill bit that uses a wedged sleeve insert to retain the cutter. The cutter is composed of a tapered carbide button that is inserted into a straight hole where the sleeve is forced around the cutter. The cutter is retained in the drill bit by the shear strength of the wedged sleeve. If properly secured, subsequent cutter removal and replacement can be extremely difficult.
U.S. Pat. No. 3,771,612 teaches a rock drill bit that uses a mounting device to secure the cutter in a recess in the drill bit. The mounting device being a onepiece goblet-shaped unit consisting of a sleeve for receiving the cutter and a pedestal for supporting the cutter. Release and replacement of the cutter requires the application of a significant axial force sufficient to deform the mounting device, allowing for cutter release.
U.S. Pat. No. 3,820,849 teaches a rock drill bit that uses a wedge insert to retain the cutter. The cutter has a T-shaped base end that is secured in a recess in the drill bit, restricting vertical movement of the cutter, and a wedge is then placed in the same recess to restrict lateral movement of the cutter. While this design allows for easier cutter replacement, secure retention of the cutter is not possible in all drilling orientations.
U.S. Pat. No. 4,271,917 teaches the use of a locking pin to secure the cutter. The locking pin is placed into a small bore alongside the cutter recess and is hammered into place. Use deforms the locking pin, which more tightly secures the cutter, yet makes subsequent cutter replacement difficult.
U.S. Pat. No. 5,322,351 teaches the use of a trapezoid-shaped wedge with a central locking bolt, in combination with a specially designed recess, to secure a cutter mounting assembly onto a rotary cutter.
U.S. Pat. No. 5,906,245 teaches a resilient split ring to secure a cutter, with a corresponding annular ring, into a drill bit recess. Rapid installation and removal of the cutter is accomplished by overcoming the tension in the resilient ring when inserting or removing the cutter. Failure of the resilient ring results in failure of attachment or greater difficulty in removing a worn or damaged cutter.
Despite this diversity of prior art, there remains a need for an apparatus and method for retaining cutters on a drill bit body that allows for quick and easy cutter installation and replacement as well as for allowing a worn cutter to be reused rather than replaced.
According to the invention, there is provided a drill bit assembly wherein a drill bit cutter is releasably retained. The drill bit assembly provides locking of the cutter in place on the drill bit body, yet permits easy removal when necessary to adjust or replace the cutter.
According to one aspect of the invention, a drill bit device or assembly is provided which comprises: a drill bit body with a first cavity formed therein; a second cavity formed therein adjacent to, and intersecting one side of, the first cavity; a disc-shaped cutter received in the first cavity; a wedge-shaped cutter lock element received in the second cavity for releasably retaining the cutter in the first cavity; and a removable fastener for releasably retaining the cutter lock element in said second cavity.
Preferably, the first cavity is of a semi-cylindrical shape with a concave bottom surface disposed such that when the disc-shaped cutter and the wedge-shaped cutter lock element are received in the first and second cavities, respectively, the cutter lock element securely retains the disc-shaped cutter in place.
The second cavity preferably comprises an upper wedge-shaped portion and a smaller lower cylindrical portion. Advantageously, the upper wedge-shaped portion is defined by at least two side surfaces that are tapered inward toward a bottom surface. Advantageously, the lower cylindrical portion intersects the bottom surface of the wedge-shaped portion and has a cross-sectional area less than the area of the bottom surface of the wedge-shaped portion. In one embodiment, the cylindrical portion includes screw threading.
In an important implementation, the disc-shaped cutter comprises a substrate disc with a layer of preferably polycrystalline diamond (PCD) bonded to at least one of the two planar surfaces. The substrate is typically formed from a hard material such as tungsten carbide.
In one embodiment, the wedge-shaped cutter lock comprises a wedge that is trapezoidal in cross-section, having four sides, a top surface and bottom surface, with at least one of the cutter lock sides tapering inward towards the bottom surface. A hole extends through the wedge from the top surface to the bottom surface. The shape of the hole is complementary to the shape of the fastener used with the hole.
The fastener comprises a removable mechanical fastener, preferably a screw.
Further features and advantages of the present invention will be set forth in, or apparent from, the drawings and detailed description that follows.